The treatment and monitoring of water is a critical societal need. Approximately three percent (3%) of all electricity produced in the United States is consumed by wastewater treatment infrastructure. Of the electricity produced, approximately one and one-half percent (1.5%) is used in the actual treatment of wastewater. Anaerobic digestion (AD) can be used to treat more concentrated wastewater streams while generating biogas (comprised of methane, hydrogen and some carbon dioxide).
AD is a well understood process that reduces energy-intensive aeration needs and leads to a net reduction of bio-solids (sludge). Energy in biogas produced from anaerobic digestion can then be tapped using generators, fuel cells, or other devices. In recent years, biogas from anaerobic digestion and natural gas has emerged as an important partial solution to our energy needs. Burning methane removes a potential greenhouse gas and generates both heat and electricity for use on-site or sale back to the grid. If the gas is generated through anaerobic digestion, the electricity will be carbon neutral.
Unfortunately AD has certain disadvantages including: (1) relatively high retention times and large foot-print required; (2) high concentrations of CO2 in biogas; (3) a requirement for pH control (addition of caustic); (4) the complex association of microbial partners required for an AD system to function optimally results in the system being sensitive to changes in temperature, pH and influent organics and; (5) elevated levels of hydrogen sulfide (H2S) in the biogas. Current anaerobic digestion methods are typically ineffective for treating water to levels low enough for environmental release. The cumulative effect of these drawbacks, keep the cost of wastewater treatment high, which, thereby affects applications for a range of industries and municipalities.
One of the most important problems associated with AD is elevated levels of hydrogen sulfide (H2S) in the biogas. Concentrations typically range between 0.1% and 2% depending on the feedstock. H2S is odorous, very corrosive to internal combustion engines, and considered an air pollutant, and thus must be removed (or scrubbed) from biogas prior to combustion. Therefore, there is a great need for low-cost H2S removal technologies that function effectively at small to mid-scale. A range of solutions currently exist to scrub natural gas of these acid compounds. For example, H2S can by reduced to very low levels by wet scrubbers using caustic and chlorine or sodium hypochlorite. However, the chemical storage, metering, and control equipment all add to the cost of a scrubber. The chemicals are hazardous, and the amounts required to treat H2S are quite high, so the cost of operating the scrubber can quickly add up.
Recent discoveries have shown that novel “electrogenic” organisms are capable of oxidizing and reducing a range of substrates including acids in gases and liquid streams, while maintaining electrical contact with electrodes. Often, the ability for electrogenic microbes to donate or accept electrons at electrodes enhances desired chemical reactions taking place, particularly, though not solely, with respect to reaction rates, reaction control, and operating ranges. The downside of such biological filtration methods is that is can be slower than chemical scrubbing and often requires upkeep including addition of nutrients and the influx of significant amounts of air into the system. Biological solutions would thus benefit from increased speeds of operations and operating ranges.
Thus, there is a critical need for cheaper and more energy efficient wastewater treatment technologies, as well as improved methods for scrubbing acidic compounds and/or CO2 from biogas.